U.S. patent number 5,658,371 [Application Number 08/554,175] was granted by the patent office on 1997-08-19 for single bed pressure swing adsorption process for recovery of oxygen from air.
This patent grant is currently assigned to Praxair Technology, Inc.. Invention is credited to Timothy Mark Aaron, John Harry Fassbaugh, Herbert Raymond Schaub, James Smolarek.
United States Patent |
5,658,371 |
Smolarek , et al. |
August 19, 1997 |
Single bed pressure swing adsorption process for recovery of oxygen
from air
Abstract
A pressure swing adsorption process for the recovery of oxygen
from air improves upon a prior art process by depressurizing the
adsorbent bed within an adsorbent vessel to an intermediate
pressure by releasing void space gas from the product end of the
vessel to a low purity oxygen tank while concurrently evacuating
the adsorbent vessel from the feed end. This action enables an
increased speed of depressurization and a reduction of the cycle
time. Further, the adsorbent bed is repressurized to an
intermediate pressure from the product outlet end with gas from the
low purity oxygen tank, while concurrently pressurizing the
adsorbent vessel from the input feed end. This action increases the
load time fraction for a feed/vacuum blower. Further, oxygen is
introduced to the product end of the adsorbent bed vessel from a
high purity oxygen tank (which provides product to downstream
applications) while concurrently, air is introduced to the feed end
of the adsorbent bed within the vessel. This enables an increased
speed of increase in pressure in the bed from the intermediate
desorption pressure. Next, a reduction in pressure ratio during the
cycle elevates the pressure within the desorption vessel to
approximately 7 psia, thereby reducing the evacuation requirements
placed on the feed/vacuum blower. The above actions enable
achievement of efficiency improvements on the order of 10%.
Inventors: |
Smolarek; James (Boston,
NY), Schaub; Herbert Raymond (East Amherst, NY),
Fassbaugh; John Harry (Elma, NY), Aaron; Timothy Mark
(Williamsville, NY) |
Assignee: |
Praxair Technology, Inc.
(Danbury, CT)
|
Family
ID: |
24212321 |
Appl.
No.: |
08/554,175 |
Filed: |
November 6, 1995 |
Current U.S.
Class: |
95/101; 95/103;
95/105; 95/130; 95/102 |
Current CPC
Class: |
B01D
53/0476 (20130101); B01D 53/053 (20130101); B01D
53/0446 (20130101); B01D 2259/401 (20130101); B01D
2259/40052 (20130101); B01D 2259/40079 (20130101); B01D
2256/12 (20130101) |
Current International
Class: |
B01D
53/053 (20060101); B01D 53/047 (20060101); B01D
053/053 () |
Field of
Search: |
;95/95-105,130 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
1-155926 |
|
Jun 1989 |
|
JP |
|
1-242120 |
|
Sep 1989 |
|
JP |
|
Primary Examiner: Spitzer; Robert
Attorney, Agent or Firm: Follett; Robert J.
Claims
What is claimed is:
1. A pressure swing adsorption process for the recovery of oxygen
from air, said process comprising the steps of:
(a) introducing feed air to a feed end of an adsorbent vessel which
includes a single bed of adsorbent material capable of selectively
adsorbing nitrogen, the pressure in said bed of adsorbent material
increasing from an intermediate adsorption pressure to an upper
adsorption pressure as a result of introduction of said air;
(b) introducing additional quantities of feed air to the adsorbent
vessel at the upper adsorption pressure, with oxygen passing from a
product end of the adsorbent vessel to a high purity oxygen tank at
said upper adsorption pressure;
(c) depressurizing said bed of adsorbent material to said
intermediate pressure by release of void space gas from the product
end of said bed to a low purity oxygen tank;
(d) further depressurizing the bed of adsorbent material from said
intermediate pressure to a lower desorption pressure by evacuation
of additional quantities of gas from said feed end;
(e) passing a stream of void space gas from the low purity oxygen
tank to the product end of the bed as a purge gas to displace
desorbed nitrogen from the bed of adsorbent material at the lower
desorption pressure, the displaced desorbed nitrogen being
discharged from the feed end of said bed;
(f) introducing void space gas from said low purity oxygen tank to
the product end of the bed while concurrently introducing air to
said feed end of said bed, to cause an increase in pressure in the
bed from the lower desorption pressure to an intermediate
desorption pressure; and
(g) passing additional quantities of feed air to the feed end of
said bed to raise the pressure therein, as cyclic operation of
steps (a)-(f) above is continued.
2. The process as recited in claim 1 and including, in step (a),
passing oxygen from the product end of the adsorbent vessel to the
high purity oxygen tank at a pressure approaching said upper
adsorption pressure, with or without the introduction of additional
quantities of feed air to the adsorbent vessel at the upper
adsorption pressure, and passage of oxygen from the product end of
the adsorbent vessel to said high purity oxygen tank at said upper
adsorption pressure, in said step (b).
3. The process as recited in claim 2 and including the introduction
of additional quantities of feed air to the adsorbent vessel at the
upper adsorption pressure in step (b).
4. The process as recited in claim 2 without the introduction of
additional quantities of feed air to the adsorbent vessel at the
upper adsorption pressure in step (b).
5. The process as recited in claim 1, and including, following step
(f), introducing oxygen from said high purity oxygen tank to the
product end of said bed of adsorbent material, while concurrently
introducing air to said feed end of said bed, to enable an increase
in pressure in the bed from the intermediate desorption
pressure.
6. The process as recited in claim 2, and including, after step
(c), further depressurizing said bed of adsorbent material by
release of void space gas from the product end of said bed to the
low purity oxygen tank, while concurrently evacuating said
adsorbent vessel from the feed end thereof.
7. The process as recited in claim 1, and including, after step
(f), introducing oxygen from said high purity oxygen tank to the
product end of said bed of adsorbent material, while concurrently
introducing air to said feed end of said bed, to enable an increase
in pressure in the bed from the intermediate desorption
pressure.
8. The process as recited in claim 7, wherein said lower desorption
pressure is within a range of about 5-9 psia.
9. The process as recited in claim 7, wherein said lower desorption
pressure is about 6.5 psia.
10. The process as recited in claim 9, wherein said upper
adsorption pressure lies within a range of about 20-24 psia.
11. The process as recited in claim 9, wherein said upper
adsorption pressure is about 22 psia.
12. The process as recited in claim 1, and including, after step
(c), further depressurizing said bed of adsorbent material by
release of additional quantities of void space gas from the product
end of said bed to the low purity oxygen tank, while concurrently
evacuating said adsorbent vessel from the feed end thereof.
13. A pressure swing adsorption process for the recovery of oxygen
from air, said process comprising the steps of:
(a) introducing feed air to a feed end of an adsorbent vessel
including a single bed of adsorbent material capable of selectively
adsorbing nitrogen, the pressure in said bed of adsorbent material
increasing from an intermediate adsorption pressure to an upper
adsorption pressure as a result of introduction of said air;
(b) introducing additional quantities of feed air to the adsorbent
vessel at the upper adsorption pressure, with oxygen passing from a
product end of the adsorbent vessel to a high purity oxygen tank at
said upper adsorption pressure;
(c) depressurizing the bed to said intermediate pressure by a
release of void space gas from the product end of the bed to a low
purity oxygen tank;
(d) further depressurizing the bed from said intermediate pressure
to a lower desorption pressure by evacuation of additional
quantities of gas from said feed end;
(e) passing a stream of void space gas from the low purity oxygen
tank to the product end of the bed as a purge gas to displace
desorbed nitrogen from the bed at the lower desorption pressure,
the displaced desorbed nitrogen being discharged from the feed end
of the bed;
(f) introducing void space gas from said low purity oxygen tank to
the product end of the bed while concurrently introducing air to
said feed end of said bed, to cause an increase in pressure in the
bed from the lower desorption pressure to an intermediate
desorption pressure;
(g) introducing oxygen from said high purity oxygen tank to the
product end of the bed while concurrently introducing air to said
feed end of said bed, to enable an increase in pressure in the bed
from the intermediate desorption pressure; and
(h) passing additional quantities of feed air to the feed end of
said bed to raise the pressure therein, as cyclic operation of
(a)-(g) above is continued.
14. The process as recited in claim 13 and including, in step (a),
passing oxygen from the product end of the adsorbent vessel to the
high purity oxygen tank at a pressure approaching said upper
adsorption pressure, with or without the introduction of additional
quantities of feed air to the adsorption vessel at the upper
adsorption pressure, and passage of oxygen from the product end of
the adsorbent vessel to said high purity oxygen tank at said upper
adsorption pressure, in said step (b).
15. The process as recited in claim 14, and including, after step
(c), further depressurizing said bed of adsorbent material by
release of additional quantities of void space gas from the product
end of said bed to the low purity oxygen tank, while concurrently
evacuating said adsorbent vessel from the feed end thereof.
16. The process as recited in claim 13, and including, after step
(c), further depressurizing said bed of adsorbent material by
release of additional quantities of void space gas from the product
end of said bed to the low purity oxygen tank, while concurrently
evacuating said adsorbent vessel from the feed end thereof.
17. The process as recited in claim 13, wherein said lower
desorption pressure is within a range of about 5-9 psia.
18. The process as recited in claim 13, wherein said lower
desorption pressure is about 7 psia.
19. The process as recited in claim 18, wherein said upper
adsorption pressure lies within a range of about 20-24 psia.
20. The process as recited in claim 19, wherein said upper
adsorption pressure is about 22 psia.
Description
FIELD OF THE INVENTION
This invention relates to pressure swing adsorption systems for air
separation and, more particularly, it relates to the use of a
single bed, pressure swing adsorption system for the recovery of
oxygen from air.
BACKGROUND OF THE INVENTION
Pressure swing adsorption (PSA) processes provide a commercially
attractive approach for separating and purifying at least one
component of a feed gas mixture which contains at least one less
readily adsorbable component and at least one more readily
adsorbable component. In the case of air, the more readily
adsorbable component is typically nitrogen, and the less readily
adsorbable component is oxygen. Adsorption occurs in an adsorbent
bed at an upper adsorption pressure with the more readily
adsorbable component, e.g. the nitrogen component, thereafter being
desorbed from the adsorbent bed by reducing adsorbent bed pressure
to a low desorption pressure.
Multi-bed PSA processes are particularly effective for oxygen plant
capacities in the size range of 30,000 NCFH to 120,000 NCFH or more
of oxygen. For applications with flow requirements that fall below
this range, a single-bed pressure swing adsorption system is
desirable. In U.S. Pat. No. 5,370,728 to LaSala et al., entitled
"Single Bed Pressure Swing Adsorption System and Process", assigned
to the same Assignee as this application, a single bed PSA or VPSA
(vacuum pressure swing adsorption) process is disclosed which
utilizes a pair of external surge tanks, one such tank supplying
high purity oxygen both as the desired product and also as a purge
gas to the adsorbent bed during an evacuation step of the
processing cycle. The second surge tank collects void space gas
(i.e., low purity oxygen) which is withdrawn from the bed during
depressurization and supplies that void space gas to the adsorption
bed during a repressurization of the adsorbent bed.
In FIG. 1, a diagram is shown of the LaSala et al. system which
incorporates a single adsorbent bed for oxygen production. A
product surge tank, hereafter called "high purity oxygen tank", is
employed together with an equalization tank (hereafter called "low
purity oxygen tank") to enable product recovery and to improve
power requirements of the system. Line 1 is used to supply feed air
to a feed/vacuum blower 2 via a dust filter 13 gas silencer unit 3
and valve 4. Line 5 from feed/vacuum blower 2 connects to lines 6
and 7, with line 6 including valve 8 and outlet snubber unit 9 from
which gas is discharged through line 10. Venting of the gas stream
can also be accomplished by means of valve 15 through line 14, to
which is connected to unit 9. Line 7 includes outlet snubber unit
11, aftercooler 12 for feed gas cooling and a valve 13. Discharge
line 14 contains a valve 15. Line 16, containing valve 17, connects
to line 1 downstream of valve 4. Lines 7 and 16 both connect to
line 18 which extends from the bottom portion of an adsorbent bed
within adsorption vessel 19. From the top of adsorption vessel 19,
a line 20 extends to and connects with line 21, valve 22 and low
purity oxygen tank 23. Line 24 connects with line 20 and, via a
check valve 25, connects to high purity oxygen tank 26. As
described below, check valve 25 is not required in the constant
product make step embodiment of the subject invention. Instead,
product gas is passed through valve 29 to high purity tank 26.
Product oxygen is withdrawn from high purity oxygen tank 26 through
line 27. Line 20 also connects, via a valve 29, to line 28 and high
purity oxygen tank 26.
The operation of the system of FIG. 1, as described in the LaSala
et al. patent, involves a five step cycle having the following
sequence: (1) partial depressurization; (2) evacuation; (3) purge;
(4) partial repressurization and (5) pressurization and product
recovery. Assuming that high purity oxygen tank 26 has received its
charge of high purity oxygen from adsorbent vessel 19 and that
adsorbent vessel 19 is at an upper adsorption pressure, the
processing sequence begins to recycle by partially depressurizing
adsorbent vessel 19. Thus, valve 13 closes and valve 15 opens,
enabling feed/vacuum blower 2 to vent air to the atmosphere. Valve
22 opens and adsorbent vessel 19 begins to depressurize from the
upper adsorption pressure. Void gas is displaced from the void
volume in the adsorbent bed and is passed through line 21 to low
purity oxygen tank 23, building to a pressure of about 14.5-15
psia. The concentration of oxygen in low purity oxygen tank 23 is
typically 85-89%. Valves 8, 17 and 29. are closed during this
action, which continues until the pressure in adsorbent vessel 19
falls to an intermediate pressure, e.g., 16 psia. The approximate
cycle time for this partial depressurization step is about 4-7
seconds.
The vessel evacuation step occurs after adsorbent vessel 19 has
expelled a portion of void gas into low purity oxygen tank 23 and
the pressure in adsorbent vessel 19 has dropped to the intermediate
pressure. Valves 8 and 17 are opened and valves 4, 15, 13, 22 and
29 and check valve 25 are closed. Thus, gas in adsorbent vessel 19
is diverted out line 18, through valve 17, line 16, to the inlet of
feed/vacuum blower 2. This void gas is discharged through outlet
silencer 9 to the atmosphere. This action enables feed/vacuum
blower 2 to further evacuate adsorbent bed vessel 19 to below
atmospheric pressure.
The approximate composition of the evacuation gas, averaged over
the evacuation portion of the cycle, is 90% nitrogen and 10%
oxygen. Adsorbent vessel 19 is evacuated to below atmospheric
pressure to cause the difference in partial pressures of the
nitrogen gas in the void spaces of the adsorbent to desorb and thus
regenerate the adsorbent to prepare for a next cycle. The vessel
evacuation step takes place until the pressure in adsorption vessel
19 reaches a lower desorption pressure, e.g., approximately 5 psia.
The step time for this cycle is about 25-40 seconds.
Next, a vessel purge step occurs at the lower desorption pressure.
Valve 29 opens and a small side stream of product gas from high
purity oxygen tank 26 is diverted into the top of adsorbent vessel
19. The oxygen input sweeps away a large portion of the remaining
void gas in vessel 19 which is comprised mainly of desorbed
nitrogen. The purge gas stream displaces the desorbed gas present
in the void volume of the adsorbent vessel 19. The vessel purge
step occurs at a constant vacuum or other desorption pressure
level, with valves 8 and 17 remaining open, control valve 29 open
and all other valves closed.
When most of the desorbed gas in the void spaces of the adsorbent
in adsorption vessel 19 is replaced with the product gas (oxygen),
the processing sequence advances to a partial repressurization
step. The average length of time for the vessel purge step is
approximately 7-10 seconds.
During the partial repressurization step, valves 8, 17 and 29 are
closed and valves 4 and 15 are opened to allow feed/vacuum blower 2
to run unloaded. Control valve 22 is opened and void gas from low
purity oxygen tank 23 (that was collected during the partial
depressurization step) is used to repressurize adsorbent vessel 19
to an intermediate pressure level, e.g., 10 psia. The time for this
step is approximately 4-7 seconds.
Now that the adsorbent bed in adsorbent vessel 19 has been
partially repressurized to an intermediate pressure of about 10
psia, feed air is supplied from feed vacuum blower 2 during a
pressurization/product recovery step of the cycle. Under these
conditions, valves 4 and 13 are open and valves 8, 15, 17, 22 and
29 are closed. Check valve 25 is adjusted so that it opens when the
pressure in adsorbent vessel 19 becomes greater than the pressure
in high purity oxygen tank 26.
As feed air is introduced into adsorbent bed vessel 19, the
pressure therein increases until it is equal to that in high purity
oxygen tank 26. Check valve 25 then opens and product gas (i.e.,
oxygen) is fed to high purity oxygen tank 26. The supply of product
gas continues until the pressure at the top of adsorbent vessel 19
reaches an upper adsorption pressure, typically about 22.5 psia.
Now, high purity oxygen tank 26 is available to provide oxygen for
downstream use, independent of adsorbent vessel 19. A typical time
for this portion of the process is about 18-25 seconds.
Feed/vacuum blower 2 has a limited differential pressure
capability, and exhibits lower efficiency at high compression
ratios. Thus, it is desirable that the cycle minimize the operating
vacuum level to reduce that pressure differential. Such action
results in feed/vacuum blower 2 operating in a more efficient range
and also results in elevated suction pressure, hence, increasing
the waste capacity of the machine at higher efficiency. Further,
both high separation efficiency and high adsorbent utilization are
desirable to assure lowest power consumption and largest capacity
for a given investment.
Accordingly, it is an object of this invention to provide an
improved method of operation for a single bed, pressure swing
adsorption system.
It is another object of this invention to provide a method for
reducing differential pressure across a compressor utilized in a
single bed pressure swing adsorption system.
It is yet another object of this invention to provide an improved
single bed, pressure swing adsorption system wherein the time
required for individual portions of the operating cycle are
reduced, thereby enabling higher system efficiency.
SUMMARY OF THE INVENTION
A pressure swing adsorption process for the recovery of oxygen from
air improves upon the prior art by depressurizing the adsorbent bed
within an adsorbent vessel to an intermediate pressure by releasing
void space gas from the product end of the vessel to a low purity
oxygen tank while concurrently evacuating the adsorbent vessel from
the feed end. This action enables an increased speed of
depressurization and a reduction of the cycle time. Further, the
adsorbent bed is repressurized to an intermediate pressure from the
product outlet end with gas from the low purity oxygen tank, while
concurrently pressurizing the adsorbent vessel from the input feed
end. This action increases the load time fraction for a feed/vacuum
blower. Further, oxygen is introduced to the product end of the
adsorbent bed vessel from a high purity oxygen tank (which provides
product to downstream applications) while concurrently, air is
introduced to the feed end of the adsorbent bed within the vessel.
This enables an increased speed of increase in pressure in the bed
from the intermediate desorption pressure. Next, a reduction in
pressure ratio during the cycle elevates the pressure within the
desorption vessel to approximately 6-7 psia, thereby reducing the
evacuation requirements placed on the feed/vacuum blower. The above
actions enable achievement of efficiency improvements on the order
of 10%.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic flow diagram of a prior art embodiment of a
single bed, VPSA system for the production of oxygen from air.
FIGS. 2-7 are schematic illustrations of sequential steps of a
processing sequence employed in the prior art system shown in U.S.
Pat. No. 5,370,728.
FIGS. 8-15 are schematic illustrations of the steps of the
invention showing which steps have been added/modified as compared
to the prior art to achieve improved process efficiency,
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT
As will be apparent from the description below, a VPSA cycle in
conformance with the invention exhibits four main improvements over
the VPSA cycle disclosed in the above noted U.S. Pat. No.
5,370,728. The improvements are:
1. An elevated bottom pressure of about 6-7 psia which enables
optimization of the feed/vacuum blower.
2. A product pressurization step that is combined with both an
oxygen recovery step and a purge step to enable a reduction in
cycle time.
3. Use of an overlap oxygen recovery, pressurization/feed air step;
and
4. Use of an overlap oxygen recovery depressurization/evacuation
step.
5. Use of oxygen from the high purity oxygen tank for product
pressurization reflux and low purity oxygen from the low purity
oxygen tank for an oxygen recovery step and purge reflux
requirements.
Implementation of each of the enumerated improvements included in
the practice of the invention enables a lower pressure ratio to be
utilized for the VPSA cycle (i.e., 20-22 psia top and 6-7 psia
bottom); an approximate 2-3% improvement in efficiency resulting
from the use of low purity oxygen in the purge step; an approximate
3% efficiency improvement resulting from the use of high purity
oxygen (from the high purity oxygen tank) for product
pressurization; and an approximate 10% improvement resulting from
overlap of compression and oxygen recovery steps.
The improved VPSA cycle is optimized at about 90-93% oxygen product
purity. The VPSA cycle can be made to operate at purities as high
as 95% with a reduction in efficiency. The VPSA cycle can also
operate at lower product purities, i.e., 80%-90%. The VPSA cycle
time may vary depending on adsorbent inventory, with an optimum
cycle time being 50 seconds (which may very from 30-70 seconds
depending on bed size).
The VPSA cycle of the invention operates at a higher bottom
pressure than conventional cycles. This not only improves
efficiencies over the entire operating range, but also results in a
higher average suction pressure which more closely balances feed
and vacuum requirements from a common blower.
The VPSA cycle, employing overlapped oxygen recovery/compression
steps, places the blower into a feed or vacuum function while
simultaneously completing the oxygen recovery step. This results in
gas flow into and out of the adsorber bed from both the top and
bottom of the adsorber bed vessel. This action improves the
utilization of the compression equipment and results in increased
capacity for a given size adsorber bed and blower.
Because of an elevated bottom pressure, there is an increased need
for oxygen reflux in the cycle. An oxygen purge step, in
combination with the product pressurization step provides this
increased oxygen requirement. The oxygen purge requirement is
provided by void gas from the low purity oxygen surge tank. Product
pressurization gas is provided by high purity oxygen from the high
purity oxygen tank. As above stated, the low purity oxygen reflux
void gas is used in the purge step and oxygen recovery
pressurization steps. The high purity oxygen is used last in the
product pressurization step. This action results in optimal use of
the oxygen reflux by introducing the higher purity oxygen just
before the product make step, thus eliminating purity anomalies
often associated with a start of the product make step. Lastly, the
VPSA cycle does not require a pressure blow down step which results
in reduced oxygen recovery.
Turning now to FIGS. 2-7, each thereof is a schematic of the
individual steps described above with respect to the prior art VPSA
cycle disclosed in U.S. Pat. No. 5,370,728. FIGS. 8-15 are the
steps of a single bed VPSA cycle employing the invention. Prior to
describing in detail the comparative steps shown in the FIGS.,
Tables 1 and 2 below illustrate the step times, start pressures and
end pressures of each of the steps of the cycle for both the prior
art process taught by U.S. Pat. No. 5,370,728 and for a process
incorporating the invention. Note that the '728 patent's cycle
requires approximately 65 seconds, whereas the cycle employing the
invention requires approximately 49 seconds. Further, the maximum
and minimum pressures employed in the cycle described in the '728
patent are 22.5 psia and 5 psia whereas they are 22 psia and 7 psia
in the cycle according to the invention.
TABLE 1 ______________________________________ Single Bed VPSA
Cycle Start End Step Time Pressure Pressure U.S. Pat. No. 5,370,728
Step (seconds) (psia) (psia) ______________________________________
1. Equalization-falling pressure 4 22.5 16 2. Evacuation 30 16 5 3.
Purge 7 5 5 4. Equalization-raising pressure 4 5 9.5 5. Feed 6 9.5
20 6. Feed make product 14 20 22.5 Total Time 65
______________________________________
TABLE 2 ______________________________________ Single Bed VPSA
Cycle Start End Step Time Pressure Pressure Invention Step
(seconds) (psia) (psia) ______________________________________ 1a.
Oxygen recovery-depressurization 3 22 18 1b. Oxygen
recovery/overlap 1 18 16 evacuation 2. Evacuation 20 16 7 3. Purge
6.5 7 9 4a. Oxygen recovery/overlap feed 1.5 9 13 4b. Product
pressurization overlap 4 13 18 feed 5. Feed 4 18 20 6. Feed make
product 9 20 22 Total Time 49
______________________________________
FIG. 2 illustrates step 1 of the cycle described in the LaSala et
al. patent wherein adsorbent vessel 19 depressurizes, enabling void
gas to pass to low purity oxygen tank 23. FIG. 8 illustrates
depressurization step 1a as performed by the invention which occurs
after adsorbent vessel 19 has reached the maximum adsorption
pressure of approximately 22 psia and product make is complete.
Feed/vacuum blower 2 is unloaded by venting it to the atmosphere
and adsorbent vessel 19 is depressurized from 22 psia to 18 psia
(end pressures of 14-20 psia can be used). The top gas is oxygen
rich and is delivered to low purity oxygen tank 23 to be used as
reflux later in the cycle. The purity of this gas starts at the 90%
product quality level and falls as the adsorption front advances
towards the top of the adsorbent bed within adsorbent vessel 19.
The step time is 2-4 seconds.
As shown in FIG. 9, the process of the invention moves to step 1b,
wherein the pressure in the adsorbent vessel 19 has fallen to about
18 psia. At such time, feed/vacuum blower 2 begins removing waste
gas from the bottom of adsorbent vessel 19. The oxygen recovery
depressurization flow from the top of adsorbent vessel 19 continues
until the pressure level in the adsorbent bed is equal to the
pressure in low purity oxygen tank 23. The cut off pressure for
this step is about 15-17 psia and the step time is 1-2 seconds.
As shown in FIG. 3, step 2 of the prior art system next evacuates
adsorbent vessel 19 down to its end pressure of approximately 5
psia. As shown in FIG. 10, step 2 of the invention also performs
the identical step, but it only needs to reduce the pressure in
adsorbent vessel 19 from 16 to 6-7 psia. The step time is
approximately 20 seconds as compared to the step time in FIG. 3 of
30 seconds (to get to 5 psia). During step 2, adsorbent bed vessel
19 is closed at the top and vacuum pumped to remove waste nitrogen
from the vessel. Variations in the bottom pressure can be achieved,
but at the expense of efficiency.
Step 3 in both the prior art (FIG. 4) and the invention (FIG. 11),
involves the purging of adsorbent vessel 19. During this step,
feed/vacuum blower 2 removes waste nitrogen from the bottom of
adsorbent vessel 19 while oxygen purge gas enters the top. As
contrasted to the prior art (FIG. 4) wherein purge gas is retrieved
from high purity oxygen tank 26, in the invention, purge gas is
obtained from low purity oxygen tank 23. A preferred condition is
that the pressure level be slightly raised during the step. The
pressure level is controlled by regulating the oxygen purge flow
rate and completes the waste removal period of the cycle, with the
oxygen front falling very near the bottom of the adsorber bed
within adsorbent vessel 19. Step 3 is completed when waste purity
at the bottom of the adsorber bed begins to slowly rise, signalling
oxygen break through. The step time is 5-9 seconds.
The next step in the prior art (step 4) involves equalization and
the raising of pressure within adsorbent vessel 19 by switching the
oxygen feed from high purity oxygen tank 26 to low purity oxygen
tank 23 (FIG. 5). In FIGS. 12 and 13, the same action is
accomplished by the substeps of oxygen
recovery-pressurization/overlap feed (step 4a) and product
pressurization/overlap feed (step 4b). In step 4a, feed air is
introduced by feed/vacuum blower 2 into the bottom of adsorbent
vessel 19 and the pressure therein begins to rise. The pressure
level rises from about 9 psia to 13 psia. Oxygen reflux gas is
simultaneously introduced to the top of adsorbent vessel 19 from
low purity oxygen tank 23. The purity of this oxygen is between 85
and 90%.
The introduction of oxygen gas during step 4a (FIG. 12) increases
the utilization of feed/vacuum blower 2, maximizing capacity for
the unit. The bed pressure rises rapidly and allows the bulk air
feed to be introduced at as high a pressure as possible. The step
time of step 4a is 1-2 seconds.
In FIG. 13, step 4b continues introduction of feed air from
feed/vacuum blower 2, while oxygen reflux gas is simultaneously
introduced into the top of adsorbent vessel 19, but now from high
purity oxygen vessel 26. The pressure rises from 13 psia to 18 psia
during this step. The purity of the inlet oxygen is now 90%, or
product quality. By utilizing oxygen from high purity oxygen tank
26, reflux oxygen is raised to a level equal to that of the product
as the adsorbent bed nears the product make step. The step time for
this portion of the cycle is approximately 3-5 seconds.
As shown in FIG. 6, the prior art step 5 continues the cycle by a
feed step wherein air is introduced by feed/vacuum blower 2 into
the bottom of adsorbent vessel 19. As can be seen from FIG. 14, an
identical step 5 occurs in the invention, with the pressure feed
air being continued until the pressure in adsorption vessel 19
reaches the product make pressure. This pressure is as close to the
top pressure as possible and may vary depending on the size of high
purity oxygen tank 26. The final pressure at the end of this step
is about 19-21 psia, and the step time is 4 seconds.
Finally, the feed/make product step 6 of the prior art process
(FIG. 7) involves continuing the feed of air from feed/vacuum
blower 2 into adsorbent vessel 19 and feeding the oxygen from the
top of vessel 19 into high purity oxygen tank 26.
As shown in FIG. 15, feed/make product step 6 is utilized wherein
feed air is introduced into the bottom of adsorbent vessel 19 at or
near the top pressure. Because of the use of oxygen from the
product pressurization/overlap feed (shown in FIG. 13), there are
no purity fluctuations at the beginning of the make step. The
improved process also operates with a relatively constant pressure,
product make step, with reduced high purity reflux requirements as
compared to the original cycle. Therefore, the step time for
feed/make product step 6 is 9 seconds, as compared to 14 seconds
for step 6 shown in FIG. 7.
Step 6 can be operated in a rising pressure or relatively constant
pressure mode. A constant pressure product make system is
preferred. The oxygen separation front does not break through
during the product make step nor do any large fluctuations in
purity exist at the start of the product make due to the
introduction of product quality oxygen as pressurization gas prior
to the product make step (see FIG. 13). The pressure at the end of
step 6 is the final top pressure of 20-22 psia.
The above described cycle produces a relatively constant purity
oxygen flow during the product make period. The product is
accumulated in high purity oxygen tank 26 for continuous delivery
to a customer. The pressure level fluctuations of the oxygen
product can be controlled by sizing of tank 26 and a product oxygen
pressure control system. The accumulation of product into tank 26
also acts as a mixing tank and achieves nearly constant product
purity.
The VPSA cycle shown in FIGS. 8-15 can be made to operate without
the oxygen recovery/depressurization/overlap evacuation step shown
in FIG. 9. The overlap oxygen recovery/depressurization/overlap
evacuation step is used to optimize the vacuum pump utilization and
is affected by the size of low purity oxygen tank 23. Further,
while a top pressure of 20-22 psi and a bottom pressure of about
6-7 psi are preferred, the top and bottom pressure levels can also
be varied in the cycle. This would occur to optimize the
performance of specific adsorbents and machines (e.g., bottom
pressures of 5-9 psia and top pressures of 19-24 psia).
The purge step shown in FIG. 11 can also be varied, depending on
the bottom pressure and adsorbent characteristics. The purge step
normally occupies approximately 10% of the cycle time. By adjusting
pressures and purge flow rates, the time of the purge step can be
varied from 0 to 15% of the cycle time.
The product pressurization step shown in FIG. 13 can also be
increased or decreased to provide pressurization gas for a varying
length of time. The amount of product pressurization gas used is a
function of the size of high purity oxygen tank 26.
Lastly, the cycle can be made to operate with a constant pressure
versus rising pressure product make. The size of high purity oxygen
tank 26 and the specifics of the product oxygen flow control can be
used to alter the pressure profile during the product make step.
Cycle testing has shown that capacity increases of 3-4% are
expected with a constant pressure system. The cycle now operates
with the high and low purity tanks (26 and 23) communicating with
the adsorber via valves 29 and 22, respectively. The improved cycle
can now be operated without check valve 25 as valve 29 can be
controlled to operate as a check valve.
It should be understood that the foregoing description is only
illustrative of the invention. Various alternatives and
modifications can be devised by those skilled in the art without
departing from the invention. Accordingly, the present invention is
intended to embrace all such alternatives, modifications and
variances which fall within the scope of the appended claims.
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